Update 4:Why you don’t want to shard. by Morgon on the MySQL Performance Blog. Optimize everything else first, and then if performance still isn’t good enough, it’s time to take a very bitter medicine. Update 3:Building Scalable Databases: Pros and Cons of Various Database Sharding Schemes by Dare Obasanjo. Excellent discussion of why and when you would choose a sharding architecture, how to shard, and problems with sharding.Update 2:Mr. Moore gets to punt on sharding by Alan Rimm-Kaufman of 37signals. Insightful article on design tradeoffs and the evils of premature optimization. With more memory, more CPU, and new tech like SSD, problems can be avoided before more exotic architectures like sharding are needed. Add features not infrastructure. Jeremy Zawodny says he's wrong wrong wrong. we're running multi-core CPUs at slower clock speeds. Moore won't save you.Update: Dan Pritchett shares some excellent Sharding Lessons: Size Your Shards, Use Math on Shard Counts, Carefully Consider the Spread, Plan for Exceeding Your Shards

Once upon a time we scaled databases by buying ever bigger, faster, and more expensive machines. While this arrangement is great for big iron profit margins, it doesn't work so well for the bank accounts of our heroic system builders who need to scale well past what they can afford to spend on giant database servers. In a extraordinary two article series, Dathan Pattishall, explains his motivation for a revolutionary new database architecture--sharding--that he began thinking about even before he worked at Friendster, and fully implemented at Flickr. Flickr now handles more than 1 billion transactions per day, responding in less then a few seconds and can scale linearly at a low cost.

What is sharding and how has it come to be the answer to large website scaling problems?

Information Sources

What is sharding?

While working at Auction Watch, Dathan got the idea to solve their scaling problems by creating a database server for a group of users and running those servers on cheap Linux boxes. In this scheme the data for User A is stored on one server and the data for User B is stored on another server. It's a federated model. Groups of 500K users are stored together in what are called shards.

The advantages are:

High availability. If one box goes down the others still operate.

Faster queries. Smaller amounts of data in each user group mean faster querying.

More write bandwidth. With no master database serializing writes you can write in parallel which increases your write throughput. Writing is major bottleneck for many websites.

You can do more work. A parallel backend means you can do more work simultaneously. You can handle higher user loads, especially when writing data, because there are parallel paths through your system. You can load balance web servers, which access shards over different network paths, which are processed by separate CPUs, which use separate caches of RAM and separate disk IO paths to process work. Very few bottlenecks limit your work.

How is sharding different than traditional architectures?

Sharding is different than traditional database architecture in several important ways:

Data are denormalized. Traditionally we normalize data. Data are splayed out into anomaly-less tables and then joined back together again when they need to be used. In sharding the data are denormalized. You store together data that are used together.

This doesn't mean you don't also segregate data by type. You can keep a user's profile data separate from their comments, blogs, email, media, etc, but the user profile data would be stored and retrieved as a whole. This is a very fast approach. You just get a blob and store a blob. No joins are needed and it can be written with one disk write.

Data are parallelized across many physical instances. Historically database servers are scaled up. You buy bigger machines to get more power. With sharding the data are parallelized and you scale by scaling out. Using this approach you can get massively more work done because it can be done in parallel.

Data are kept small. The larger a set of data a server handles the harder it is to cash intelligently because you have such a wide diversity of data being accessed. You need huge gobs of RAM that may not even be enough to cache the data when you need it. By isolating data into smaller shards the data you are accessing is more likely to stay in cache.

Smaller sets of data are also easier to backup, restore, and manage.

Data are more highly available. Since the shards are independent a failure in one doesn't cause a failure in another. And if you make each shard operate at 50% capacity it's much easier to upgrade a shard in place. Keeping multiple data copies within a shard also helps with redundancy and making the data more parallelized so more work can be done on the data. You can also setup a shard to have a master-slave or dual master relationship within the shard to avoid a single point of failure within the shard. If one server goes down the other can take over.

It doesn't use replication. Replicating data from a master server to slave servers is a traditional approach to scaling. Data is written to a master server and then replicated to one or more slave servers. At that point read operations can be handled by the slaves, but all writes happen on the master.

Obviously the master becomes the write bottleneck and a single point of failure. And as load increases the cost of replication increases. Replication costs in CPU, network bandwidth, and disk IO. The slaves fall behind and have stale data. The folks at YouTube had a big problem with replication overhead as they scaled.

Sharding cleanly and elegantly solves the problems with replication.

Some Problems With Sharding

Sharding isn't perfect. It does have a few problems.

Rebalancing data. What happens when a shard outgrows your storage and needs to be split? Let's say some user has a particularly large friends list that blows your storage capacity for the shard. You need to move the user to a different shard.

On some platforms I've worked on this is a killer problem. You had to build out the data center correctly from the start because moving data from shard to shard required a lot of downtime.

Rebalancing has to be built in from the start. Google's shards automatically rebalance. For this to work data references must go through some sort of naming service so they can be relocated. This is what Flickr does. And your references must be invalidateable so the underlying data can be moved while you are using it.

Joining data from multiple shards. To create a complex friends page, or a user profile page, or a thread discussion page, you usually must pull together lots of different data from many different sources. With sharding you can't just issue a query and get back all the data. You have to make individual requests to your data sources, get all the responses, and the build the page. Thankfully, because of caching and fast networks this process is usually fast enough that your page load times can be excellent.

How do you partition your data in shards? What data do you put in which shard? Where do comments go? Should all user data really go together, or just their profile data? Should a user's media, IMs, friends lists, etc go somewhere else? Unfortunately there are no easy answer to these questions.

Less leverage. People have experience with traditional RDBMS tools so there is a lot of help out there. You have books, experts, tool chains, and discussion forums when something goes wrong or you are wondering how to implement a new feature. Eclipse won't have a shard view and you won't find any automated backup and restore programs for your shard. With sharding you are on your own.

Implementing shards is not well supported. Sharding is currently mostly a roll your own approach. LiveJournal makes their tool chain available. Hibernate has a library under development. MySQL has added support for partioning. But in general it's still something you must implement yourself.

I am not sure what you mean about live journal. It goes to a page on this site which references two danga.com sites. Oh I see, memcached goes to a category link which doesn't include memcached. The hover text does include the link, but I'll add it in. Good catch. Thanks.

You won't have the problems with replication overhead and lag because you are writing to a appropriately sized shard rather than a single master that must replicate to all its slaves, assuming you have a lot of slaves.

You will still replicate within the shard, but that will be more of a fixed reasonable cost because the number of slaves will be small.

Google replicates content 3 times, so even in that case it's more of a fixed overhead then chaining a lot of slaves together.

I think a lookup table is the most flexible option. It allows for flexible shard assignment algorithms and you can change the mapping when you need to. Here a few other ideas. I am sure there are more.

http://highscalability.com/flickr-architecture">Flickr talks about a "global ring" that is like DNS that maps a key to a shard ID. The map is kept in memcached for a 1/2 hour or so. I bought Cal Henderson's book and it should arrive soon. If he has more details I'll do a quick write up. Users could be assigned to shards initially on a round robin basis or through some other whizzbang algorithm.

You could embed shard IDs into your assigned row IDs so the mapping is obvious from the ID.

You could hash on a key to a shard bucket.

You could partition based on on key values. So users with names starting with A-C go to shard1, that sort of thing. MySQL has a number of different partition rules as examples.

Attacking sharding from the application layer is simply wrong. This functionality should be left to the DBMS. Access from the app layer would be transparent and it would be up to the DB admin to configure the data servers correctly for sharding to automatically and transparantly scale out across them.

If you are implementing sharding from the app layer you are getting yourself in a very tight corner and one day will find out how stuck you are there. This is the cornerstone of improper delegation of the functionalities in a multi-tier system.

>How do you partition your data in shards? What data do you put in which shard? Where do comments go? Should all user data really go together, or just their profile data? Should a user's media, IMs, friends lists, etc go somewhere else? Unfortunately there are no easy answer to these questions.

I have exactly the question to ask..I've refered the architectures of LiveJournal and Mixi, both of which introduce shards.Howerver, I saw a "Global Cluster" which store meta informations for other clusters. By doing this we get an extreme heavy cluster, it must handle all the cluster_id user_id metas and lost the advantage of sharding...Is it?The other way, partition by algorithms on keys, is difficult in transition.

So, could you give me some advice? Thank you very much for sharing experiences :)

Yeah, you reminded me! I have a little doubts on this before, when I think the table would be terribly huge as all the table records on other clusters are all gathered in this table, and all queries should first refer to this cluster.Maybe I can use memcached clusters to cache them. Thank you :)

I agree with the above post questioning the placement of partitioning logic in the application layer. Why not write the application layer against a logical model (NOT a storage model!) and then just engineer the existing data storage abstraction mechanism (DBMS engine) such that it will handle the partitioning functionality in a parallel manner? I would be very interested to see a study done comparing the architectures of this sharding concept against a federated database design such as what is described on this site http://www.sql-server-performance.com/tips/federated_databases_p1.aspx

> I would be very interested to see a study done comparing the > architectures of this sharding concept against a federated database design

Most of us don't have accesses to a real affordable federated database (parallel queries), so it's somewhat a moot point :-) And even these haven't been able to scale at the highest levels anyway.

The advantage of partitioning in the application layer is that you are not bottlenecked on the traffic cop server that must analyze SQL and redistribute work to the proper federations and then integrate the results. You go directly to where the work needs to be done.

I understand the architectural drive to push this responsibility to a logical layer, but it's hard to ignore the advantages of using client side resources to go directly to the right shard based on local context. What is the cost? A call to library code that must be kept in sync with the partitioning scheme. Is that really all that bad?

Sorry, this isn't new, except maybe to younger programmers. I've been using this approach for many years. Federated databases with horizontally partioned data is old news. Its just not taught as a standard technique for scaling, mostly because scaling isn't taught as a core subject. (Why is that, do you suppose? Too hard to cover? Practical examples not practical in an academic setting?)

The reason this is getting attention now is a perfect storm of cheap hardware, nearly free network connectivity, free database software, and beaucoup users (testers?) over the 'net.

Great content and great site... except for the worshipful adoration of these young teams who seem to think they've each discovered America.

I could go on at length about Flickr's performance troubles and extremely slow changes after their early success, Twitter's meltdowns and indefensible defense of the slowest language in wide use on the net, and old-school examples of horizontal partitioning (AKA "sharding" heh), but I'll spare you. This cotton candy sugary sweet lovin' of Web 2.0 darlings really is a bit tiresome.

Big kudos though on deep coverage of the subject matter between the cultic chants. :-D

Fanball.com has been using this technique for its football commissioner product for years. As someone else commented, it used to be called horizontal partitioning back then. Does it cost less if it's called sharding? :-)

Could it be the fact that people are pulling this off with mysql and berkeleydb that is making horizontal partitioning interesting? When you compare two solutions, one using an open source database and one using a closed source database, is one solution more inherently scalable? Well all things being equal performance wise, its nice to not have to do a purchase order for the closed source software, so I would say that is why this is getting all the 'hype'. Old school oracle/mssqlserver patronizing DBAs are getting schooled by non-dbas who are setting up the *highest* data throughput architectures and not using sql server or oracle. That is why this is getting high visibility.

Believe or not many people still say mysql/berkeleydb is a toy outside of some of the major tech hubs. Stories like this are what make people, especially dbas, listen. The only recourse is 'I have done that before with xxx database'. Well you should be the one that suggests doing it with the open source 'toy' database then, if you are so good.

In my experience there are many old-school DBAs that are in denial that this kind of architecture is capable of out performing their *multi-million dollar oracle software purchase decisions* and they don't want to admit it.

What I'm most interested in relating to Shards is people's thoughts and experience in migrating TO a shard approach from a single database, and moving (large amounts of) data around from shard to shard. In particular - strategies to maintain referential integrity as we're moving data by a user.

As well, should you need to query data joining user A and user B which both reside on different shards - what approaches people see as fit?